Research studies suggest that when postmenopausal women are treated with hormone (estrogen) replacement, they have less coronary disease than if they are not. This study is designed to see whether treatment of women who have coronary disease serious enough to require bypass surgery will show less progression of disease or less new disease developing in the bypass grafts if treated with estrogen over a 3.5 year period. The purpose of the study is to determine if estrogen replacement therapy will slow the progression of narrowings in saphenous vein grafts used for coronary artery bypass surgery in postmenopausal women with heart disease. The subjects at this site will be 30 postmenopausal women who have recently undergone bypass surgery and who do not have any contraindications for bypass surgery. We hope to learn more about whether estrogen can prevent future heart attacks or prevent the need for future procedures. In addition, information about the bypass grafts can be given to the participants and their physicians, which may alter their ongoing therapy.